(Revised Abstract) Overweight and obesity are major health problems in the United States, affecting more than 50% of adults. The long-term consequences of being overweight or obese include increased mortality and increased morbidity from a variety of associated disease states. Short-term weight loss has been demonstrated to ameliorate obesity-related metabolic abnormalities and cardiovascular disease risk factors. However, observational studies have raised concerns about negative effects of weight loss and weight cycling over the long term, including increased mortality. Look AHEAD (Action For Health in Diabetes) is a 2-armed randomized controlled clinical trial studying overweight and obese volunteers with type 2 diabetes at 16 clinical centers. Approximately 5,000 volunteers with type 2 diabetes aged 45-75 years with body mass index > 25 kg/m will be recruited, including approximately equal numbers of men and women and at least 33% participants from ethnic minority groups. The primary objective of Look AHEAD is to examine the long-term health effects of an intensive Lifestyle Intervention designed to achieve and maintain weight loss by decreased caloric intake and increased physical activity. The Lifestyle intervention is implemented with individual supervision and group sessions and aims to achieve at least a 7% decrease in weight from baseline and 175 minutes per week in physical activity on average and to sustain these goals. This program will be compared to a control condition involving a program of Diabetes Support and Education. The primary hypothesis is that the incidence rate of the first post-randomization occurrence of a composite outcome, which includes cardiovascular death (including fatal myocardial infarction and stroke), non-fatal myocardial infarction, and non-fatal stroke, over a planned follow-up period of up to 11.5 years will be reduced among participants assigned to the Lifestyle Intervention compared to those assigned to Diabetes Support and Education. The study is projected to have 90% probability of detecting an 18% difference in this primary outcome between the 2 groups. A composite secondary outcome of all deaths, CVD events, and CVD procedures has been defined. Additional outcomes include: diabetes control and complications, fitness, general health, health-related quality of life, and psychological outcomes. The cost and cost effectiveness of the Lifestyle Intervention relative to Diabetes Support and Education will be assessed.